The invention relates generally to a method and system for determining reference speed in a vehicle and in particular to a method and system for determining a reference speed in a locomotive driven by AC motors. Locomotives used for heavy haul applications generally are equipped with speed sensors connected to individual traction motors. Information from these speed sensors are compared with a reference speed estimating the true ground speed for obtaining the maximum adhesion and for wheel slip/slide protection. The estimate of ground speed, or reference speed, relies on individual motor speed sensors along with other ground speed measurements such as radar-based ground speed measurements. The use of radar to estimate ground speed has disadvantages due to cost, accuracy and misoperation due to ground conditions and other environmental conditions. Accordingly, there is a need in the art for a technique for estimating ground speed which does not use radar.